The invention is directed to a hydrophobic filler mixture, a process of producing it and its use as a filler in RTV (room temperature vulcanization) and LTV (low temperature vulcanization) silicone rubber.
RTV-silicone rubbers generally are produced in two different types of reaction systems. The first general type is the so-called two component system in which the crosslinking element is an alkyl silicate or an alkoxy silane. This system above all is characterized by a spontaneous vulcanization if the crosslinking element and catalyst are mixed.
The second general type is the so-called single component system in which a polymer, a crosslinking element and in a given case a catalyst are mixed together in the absence of moisture. This mixture cross-links in the subsequent contact with moisture in which case above all, the atmospheric mositure is used (see British Pat. No. 1,110,429. The entire disclosure of British Pat. No. 1,110,429 is hereby incorporated by reference).
It is known to employ in single component or two component RTV-silica rubber compositions as fillers precipitated silica or pyrogenically produced silica in which case the silicas, optionally with linear siloxanes, cyclic siloxanes, halosilanes, silanols or alkoxysilanes are hydrophobized (see Great Britain Pat. No. 1,110,429).
These fillers, however, in part exhibit disadvantages which can negatively effect the quality of the RTV silicone rubber composition. Thus it is possible that these fillers impart a lower rigidity as well as insufficient storage stability to the silicone rubber composition and the hardened silicone rubber compositions exhibit unsatisfactory mechanical properties.
Besides in using the relatively easily volatile hydrophobizing agents there exist work place hydiene considerations.